Diaphragm Construction

Horizontal diaphragms are composed of:

  • wood-based sheathing (typically plywood or oriented strand board (OSB))
  • wood framing members (e.g.- sawn lumber joists/rafters, trusses, I-joists)
  • fasteners (typically nails or staples) for attaching sheathing to framing members


Lumber blocking (short lengths of lumber) may also be provided between framing members beneath panel joints. The presence of blocking facilitates the attachment of sheathing to framing at all panel edges. Since blocked diaphragms have a greater concentration of fasteners around the perimeter of each sheathing element, blocked diaphragms provide greater shear capacities than unblocked diaphragms.

Here is a plan view (top view) of the framing members for a wood floor diaphragm.

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The higher unit shear capacities for blocked diaphragms are readily apparent in the Allowable Shear Tables for Wood Structural Panel Diaphragms published in the following documents:

  • Diaphragms and Shear Walls - Design/Construction Guide, APA-The Engineered Wood Association
  • Structural-Use Panel Shear Wall and Diaphragm Supplement to the Allowable Stress Design Manual for Engineered Wood Construction, APA-The Engineered Wood Association
  • 2000 International Building Code (Table 2306.3.1)
  • 1997 Uniform Building Code (Table 23-II-H)